Why is the outer layer of the sun’s atmosphere, the region farthest from the heat-producing core, hotter than both the lower atmosphere and the sun’s surface?
Now observations from a new generation of sun-observing spacecraft are implicating a new mechanism, one that could provide the corona with a significant portion of its heat by continually delivering hot ionized gas, or plasma, to the upper atmosphere.
Short-lived fountains of plasma known as spicules, shooting up from the sun’s chromosphere, or lower atmosphere, appear to play a role in heating the corona to searing temperatures at millions of degrees Kelvin, researchers have found. The group based its study on observations from NASA’s new Solar Dynamics Observatory, launched in 2010, and the Japanese Hinode spacecraft, which began service in 2006. Both solar observatories are capable of taking detailed images of the sun every several seconds, the kind of quick-time observation needed to identify transient or rapidly changing phenomena.
The spicules last just 100 seconds or so, shooting upward from the chromosphere at speeds of roughly 50 to 100 kilometers per second. The spicules show up brightly in wavelengths associated with temperatures much cooler than the corona, in the tens of thousands of degrees K. The plasma at those temperatures rises in jets from the chromosphere and then falls back to the surface. But the spicules are also associated in location and time of occurrence with flare-ups in other wavelengths that indicate plasma temperatures of at least 1 million to 2 million degrees K in the corona. Something, then, seems to be heating spicule plasma to exceptional temperatures as the hotter plasma rises toward the corona.
The fountains last just minutes, but the researchers estimate that they occur often enough to potentially account for a good chunk of coronal heat.
– from scientificamerican.com